Microporous and ultrafiltration membranes are well known in the particle filtration industry. The material or matrix of these membranes comprises suitable organic plastics such as nylons, polysulfones, acrylics, and the like. Their filtration mechanism is a combination of size exclusion (sieving) and absorption or adsorption on the walls of the pores inside the membrane. To be considered "microporous", the typical inner width of the membrane pores is in the range that passes macromolecules and retains particles contained in a fluid. Below this range, are "ultrafiltration" (UF) membranes which serve to filter macromolecules rather than particles, and "reverse osmosis" (RO) membranes which serve to separate ions. The smaller the pore size, the slower the rate at which a filtrate can be passed. To be useful for a particular application, the fluid flow rate through the membrane must therefore be reasonably high.
In aqueous filtration, it is desirable to have membranes that are easily wettable and that have as little leachable material as possible. Such hydrophilic membranes containing an inherently hydrophobic polyamide, polyimide or polyethersulfone polymer are described in European patent document no. 0 228 072 dated July 8, 1987.
Polyethersulfone polymer has been shown to be miscible in a common solvent (DMF or DMSO), with phenoxy resin (V. B. Singh and D. J. Walsh, J. Macromol, Sci.-Phys., B25 (1-2), 65-87, 1986). Also shown is that the melting temperature of cast films of blended PES/phenoxy resin is lowered by using more phenoxy resin (relative to PES) in the blend. Not suggested is a membrane made from such a blend nor was such a film suggested to be porous or to be useful as a filtration membrane.